1. Field of the Ivention
The present invention relates to a foil blade for use in a web-forming section of a papermaking machine, and relates more particularly to a foil blade for use in forming shoes, forming boxes, forming boards, foil boxes, vacuum foil boxes, suction foil boxes or, generally, dewatering devices used in a web-forming section of a papermaking machine. Specifically, the present invention relates to an improved foil blade that is easily installed into and removed from the aforementioned devices and; once mounted, the foil blade of the present invention does not rotate or change geometry during operation thereby preventing improper blade angles and geometry between the foil blade and a wire or fabric traveling thereover which, if present, cause operating problems and excessive wire or fabric wear.
2. Description of the Prior Art
In a papermaking machine, a traveling forming wire or fabric receives paper stock consisting of a mixture of water and pulp, typically referred to as a slurry of pulp, from a slice lip or slice area of a headbox. Once the stock mixture impinges and is received by the forming wire or fabric, the stock mixture travels along a web-forming section of the papermaking machine. As the stock mixture continues down the web-forming section, the stock mixture is transformed into a moving web of paper. Upon leaving the web-forming section, the web of paper undergoes further processing in a press section, a dryer section and finishing sections until a final paper product is produced.
Paper stock or slurry coming out of a headbox, depending on the type of paper being produced, typically has a consistency in the range of 0.05-1.5%. At the end of a web-forming section of a papermaking machine, depending on the type of paper being produced, a formed web of paper typically has a consistency of 10-30% as the web enters a press section of the papermaking machine.
A web-forming section of a papermaking machine can be of many different designs, depending on the type of paper being produced. Typical web-forming sections are referred to as Fourdriniers, twin wire formers, gap formers, or a combination of these. In order to transform a slurry of pulp having a consistency of 0.05-1.5% as the slurry leaves a headbox to a formed web having a consistency of 10-30% as the web leaves a web-forming section, a significant amount of water must be removed from the stock mixture between a headbox and a press section of a papermaking machine. Various dewatering devices are used in a web-forming section to remove water from the paper stock or pulp slurry being formed into a web. Such dewatering devices include forming shoes, forming boxes, forming boards, foil boxes, vacuum foil boxes, suction foil boxes, or the like, generally known to those skilled in the art.
Most of the dewatering devices used in a web-forming section of a papermaking machine include a foil blade. A function of such a foil blade is to enhance drainage of water from a pulp fiber-water mixture as the mixture or slurry travels between a headbox and a press section of a papermaking machine. Foil blades provide pressure impulses which affect the formation of a paper web as well as the drainage of water from a stock mixture. Foil blades can be located on both sides of a wire or forming fabric in certain formers. These formers drain water from both sides of a web. On Fourdrinier machines, foil blades are on the bottom side of the wire. As noted, foil blades are mounted to such dewatering devices as forming boards, foil boxes, forming shoes, and the like. These dewatering devices are essentially strength sections which may or may not be fitted with vacuum to assist in the drainage of water from the web being formed. Foil blades can be made from several different materials. Typically, foil blades are made of aluminum oxide, silicone nitride, and/or silicone carbide, which are generally referred to as "ceramics" by those skilled in the art. Poly is also sometimes used as a material of construction for a foil blade. A typical construction of a continuous ceramic foil blade has ceramic segments bonded into a pultruded fiberglass material. The base of the foil blade being the reinforced fiberglass and the top being the ceramic. There are also segmented, ceramic foil blades made as one continuous single ceramic piece, top and bottom, but in segments across the machine width. It is the top of the foil blade that is in contact with a forming wire or fabric. The base of the foil blade must be fastened to a dewatering device or a strength section. This is usually done by either a T-bar or dove-tail clamp design, as will be more fully explained below. The advantage of the T-bar design is quick and easy removal and installation of a continuous foil blade. The disadvantage of the T-bar design is a foil blade used with this design has a tendency to rotate during operation due to the mounting clearances required in order to slide a foil blade onto its mating T-bar mounting structure. The advantage of the dove-tail design is a solid clamp foil blade that does not rotate during operation. The disadvantage of the dove-tail design is the time and effort required to remove and install a foil blade.
A foil blade has an upper surface of soft or hardened material which is accurately positioned to form a divergent angle in the machine direction with a wire or fabric traveling over the foil blade so as to cause an area of reduced pressure between the wire or fabric and the upper surface of the foil blade to effectuate dewatering of a paper stock mixture or slurry through the wire or fabric. It is imperative in the relationship between a foil blade and a wire or fabric that a uniform positional relationship be maintained with respect to the divergent angle of the upper surface of the foil blade relative to the wire or fabric, as will be more further explained directly below.
Foil blades wear with operation of a papermaking machine and a typical arrangement permits removal of a foil blade and replacement with a reworked or fresh blade. One such arrangement provides a T-bar support device attached to a dewatering device. The T-bar support device supports a foil blade. The foil blade contains an appropriate slot which allows the foil blade to be attached to the T-bar support. The sliding relationship between the slot of a foil blade and the T-bar of a T-bar support permits removal and installation of the foil blade in a cross-machine direction. Typically, tolerances between a conventional foil blade and a mounting T-bar support are in the area of 0.008 inches to 0.020 inches loose. The loose fit tolerances between a foil blade and a T-bar support allows the foil blade to rock or rotate on the T-bar support mounting as a wire or fabric travels over the foil blade during a papermaking process, thereby changing the geometry of the angle between the upper surface of a foil blade and the wire or fabric traveling over the foil blade. A change in geometry between a foil blade and the wire or fabric traveling thereover, contributes to sheet defects, nonuniform drainage, light spots, rewet, or dirt and fiber build up on the blade with resultant possible worming or streaking of the paper sheet being produced. Additionally, an improperly positioned foil blade with respect to a wire or fabric traversing over a foil blade adversely affects the optimum pressure differential needed in order to drain water from the stock mixture to form a properly formed web in a web-forming section of a papermaking machine. A rotating foil blade of the T-bar design is also problematic because the degree of rotation adversely affects the activity and the amount of pressure pulsations transmitted to the stock. Too much activity can result in stock jump which, if excessive, can cause formation defects. Not enough activity can also result in non-uniform drainage which produces streaks and other problems known to those skilled in the art. Unfortunately, if tolerances are reduced between a foil blade and a T-bar mounting support so as to reduce the amount of rocking or rotation of the foil blade during operation, it becomes extremely difficult, if not impossible, for a paper maker to install and/or remove a foil blade.
Another arrangement which permits removal of a foil blade and replacement with a reworked or fresh blade includes a dove-tail bar support device attached to a dewatering device which supports a foil blade. In this conventional arrangement, a foil blade is provided with a dove-tail such that one end of the dove-tail bar support device receives the dove-tail end of the foil blade. The other ends of the foil blade and the dove-tail bar support are clamped together by means of screws or the like. Generally, the dove-tail arrangement rigidly fixes a foil blade in position which prevents the angle between the upper surface of the foil blade and the wire or fabric traveling thereover from changing. However, a significant drawback to the dove-tail design is that a foil blade cannot be easily removed or installed without removing the wire or fabric of the entire web-forming section. The wire or fabric must be removed in order to allow access to the screws or clamping means attaching the foil blade to the dewatering device. Generally, there is not enough space to allow access to the screws with the wire or fabric in position. Those skilled in the art will appreciate the amount of work and excessive downtime of a papermaking machine, and the cost associated therewith, that are incurred in having to remove a wire or fabric in a web-forming section of a papermaking machine in order to replace old or damaged foil blades with new foil blades.
What is needed is a foil blade which eliminates the heretofore mentioned problems. What is needed is a foil blade which is easily installed and removed from dewatering devices in a web-forming section of a papermaking machine. Additionally, what is further needed is a foil blade which is rigidly mounted to a dewatering device. Such a foil blade must be able to withstand the forces being applied against it so that the foil blade does not rock or rotate during operation resulting in a change in geometry between the blade and a wire or fabric traveling over the blade. Additionally, what is needed is a foil blade that reduces operating problems and excessive wire wear during operation.